System and method for combining multiple energy bands to improve scene viewing

ABSTRACT

A single sensor that can operate in multiple bands and display either one radiation band alone or multiple overlaid bands, using an appropriate color choice to distinguish the bands. The multiple-band sensor allows the user to look through at least one eyepiece and with the use of a switch, see scenes formed via the human eye under visible light, an II sensor, an MWIR sensor, or an LWIR sensor, either individually or superimposed. The device is equipped with multiple switching mechanisms. The first, for allowing the user to select between radiation bands and overlays, and the second, as with most thermal imaging sensors, for allowing the user to switch between “white-hot/black-hot” i.e., a polarity switch.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention pertains generally to imaging using multiple bandsof radiation and particularly to the simultaneous imaging of multiplebands of radiation to form a scene for viewing by a user.

[0003] 2. Description of Related Art

[0004] Currently, an individual seeking to view objects in dark, lowlevel light conditions and/or poor atmospheric conditions, may rely oneither image intensification sensors for the visible/near-wavelengthinfrared (“referred to hereafter as VIS/NIR”) or thermal infrared (IR)sensors. Further, within the IR range, separate detectors are necessaryin order to detect both mid-wavelength IR (“MWIR”) and long-wavelengthIR (“LWIR”). No single detection system allows an individual tosimultaneously view any two of these wavelength ranges. Each sensor,independently, has significant advantages in, for example, a widevariety of military scenarios. The IR is better at detecting all typesof items (e.g., targets) under most light level and meteorologicalconditions. Camouflage and obscurants are much less effective in thethermal band than in the VIS/NIR. However, many night missions,especially those in urban settings, favor the image intensification dueto the need to read signs (e.g., alphanumerics), see through glasswindows, and recognize and differentiate individuals. Unlike VIS/NIRsensors, thermal imagers do not detect lasers operating at 1.06 microns.This is significant because such lasers are used for target ranging anddesignation. Knowledge of such illumination by friendly or enemy forcescan be vital. In addition, image intensification sensors operating inthe VIS/NIR (i.e., 0.6 to 1.1 micron range) offer considerably betterresolution than the IR sensors.

[0005] Uncooled IR sensors (both currently existing and thoseimprovements in development) offer a low cost, low power approach tothermal imaging. Operating in the long wave infrared (LWIR: 7.5 to 14microns), uncooled thermal imaging is excellent for many militaryapplications because items of interest (e.g., enemy soldiers, vehicles,disrupted earth) almost always emit more in-band (in this case IR)energy than the background. Other applications for uncooled thermalimaging include security, hunting, monitoring and surveillance,firefighting, search and rescue, drug enforcement, border patrol andship navigation. The current uncooled devices and even those indevelopment (e.g., 640×480 with 25 micron pixels) have significantlylower resolution compared to image intensification (II) devices ordaytime telescopes.

[0006] Image intensifiers take whatever amount of light is available(e.g., moonlight, starlight, artificial lights such as street lights)and electronically intensify the light and then display the image eitherdirectly or through the use of an electronic imaging screen via amagnifier or television-type monitor. Improvements in II technology haveresulted in the GEN III OMNI IV 18-mm tube offering the equivalent ofmore than 2300×2300 pixels. II covers the visible and near infrared(VIS/NIR: 0.6 to 1.1 microns) and overcomes the LWIR limitations listedabove. However, II is limited by the ambient light available, targetsare harder to find, and camouflage/obscurants (e.g., smoke, dust, fog)are much more effective. While scientists have long seen thecomplementary nature of LWIR and II to achieve sensor fusion, mostattempts involve the use of two parallel sensors and sophisticatedcomputer algorithms to merge the images on a common display, a displaywith lower resolution than the II tube. This approach is difficult toimplement for an individual handheld sensor. Currently, for example,night operations forces often carry both II and LWIR sensors, each withdifferent fields of view and magnification, for special reconnaissance,target interdiction, and strike operations. The synergy noted above islost because the soldier cannot use the devices simultaneously.

[0007] IR imaging devices typically provide monochrome imagingcapabilities. In most situations, the ideal viewing scenario would befull color. This is of course achieved in the visible band. There arenumerous situations where a user alternates between visible band viewingscenarios and IR band viewing scenarios within the span of seconds.Current imaging devices do not allow a user to either: (a)simultaneously view both the monochrome IR image and the full colorvisible image, or (b) change between IR monochrome imaging andfull-color visible imaging without mechanically altering the imagingdevice or removing the device from the users field of view (e.g., as inthe case of an IR helmet mounted sensor).

BRIEF SUMMARY OF THE INVENTION

[0008] The obvious synergy of multiple sensor bands is difficult toachieve and totally impractical for handheld use via separate sensorslooking alternately at the same scene. The solution advanced in thisapplication is the development of a single sensor that can operate inmultiple bands and display either one radiation band alone or multipleoverlaid bands, using an appropriate color choice to distinguish thebands. The multiple-band sensor allows the user to look through at leastone eyepiece and with the use of a switch, see scenes formed via thehuman eye under visible light, an II sensor, an MWIR sensor, or an LWIRsensor, either individually or superimposed. The device is equipped withmultiple switching mechanisms. The first, for allowing the user toselect between radiation bands and overlays, and the second, as withmost thermal imaging sensors, for allowing the user to switch between“white-hot/black-hot” i.e., a polarity switch.

[0009] A further feature of the present invention is a multiple-bandobjective lens for imaging multiple bands of incoming radiation,including at least two of the visible band, the VIS/NIR band, the MWIRband or the LWIR band.

[0010] A further feature of the present invention is range focusingcapability of the sensors which, simultaneously, is from 3 meters toinfinity over a full military temperature range.

[0011] Further features of the present invention include supportingsensors and corresponding eyepiece displays as well as transmitters usedto enhance the usability and efficiency of the multiple-band sensors.For example, a digital compass, a laser range finder, a GPS receiver andIR video imagery components may be integrated into the multiple-bandsensor. The multiple-band sensor may also be equipped with an eyepiecefor facilitating zoom magnification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the Figures:

[0013]FIG. 1 is a reflective objective lens according to an embodimentof the present invention;

[0014]FIG. 2 is a combination reflective/refractive objective lensaccording to an embodiment of the present invention;

[0015] FIGS. 3(a) and 3(b) are compact monoculars according toembodiments of the present invention;

[0016] FIGS. 4(a) and 4(b) are compact binoculars according toembodiments of the present invention;

[0017] FIGS. 5(a) and 5(b) are monochromatic MTF performance graphs forNIR energy passing through a lens configuration according to anembodiment of the present invention;

[0018] FIGS. 6(a) and 6(b) are monochromatic MTF performance graphs forLWIR energy passing through a lens configuration according to anembodiment of the present invention;

[0019] FIGS. 7(a)-7(d) are objective lenses according to embodiments ofthe present invention;

[0020] FIGS. 8(a)-8(f) are objective lenses according to embodiments ofthe present invention with FIGS. 8(g) and 8(h) are representativecolor-corrected MTF performance graphs for VIS/NIR energy passingthrough lens configurations 8(e) and 8(f), respectively;

[0021] FIGS. 9(a)-9(b) are objective lenses according to embodiments ofthe present invention; and

[0022] FIGS. 10(a) and 10(d) are objective lenses according toembodiments of the present invention and FIGS. 10(b), 10(c), 10(e), and10(f) are MTF performance graphs for objective lenses 10(a) and 10(d),respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0023] In forming a multiple-band sensor, there are multiple factorsthat must be considered in order to optimize the image quality in eachband simultaneously. First, it is important to keep the f/number as fastas possible, as the performance of most sensors including, the humaneye, uncooled IR, including MWIR and LWIR, as well as II depend onefficient energy collection. Second, depending on the intended use forthe multiple-band sensor, the size and weight of the sensor is animportant factor. For example, for use as a handheld device,miniaturizing and/or compacting the components of the sensor is quite achallenge. While the size and weight requirements are less limiting fortripod mounted versions of the multiple-band sensor, they are not to beignored. Third, there is a magnification tradeoff with the field of viewthat impacts the image quality and ultimately the design of themultiple-band sensor. A wide field of view is useful in wide areacoverage, but limited in target identification and intelligencegathering. Too much magnification requires a heavy lens, which limitsthe usefulness in a handheld sensor, but is less of an issue in largerfixed (mounted) applications.

[0024] In a preferred embodiment of the present invention a singleobjective lens, similar to a long range camera lens and telescope, isused to image all incoming bands of energy. In a specific embodiment,the reflective objective lens consists of two mirrors, allowing for alonger focal length in a smaller space than a single mirror wouldrequire for the same focal length. This lens configuration shown in FIG.1 is known as a catadioptric configuration (imaging objectives usingpowered lenses and mirrors to form the image) consisting of a Cassegraintwo-mirror lens with an aperture corrector and a field lens. TheCassegrain portion of the objective lens is a telephoto configuration,which allows it to be compact. Other embodiments may be more elaboratecatadioptic set-ups to obtain the desired imaging properties of theobjective lens.

[0025] The objective lens 100 of FIG. 1 comprises a combinationcorrector lens/aperture stop 110, a primary component 115, a secondarycomponent 120, and a field lens 125.

[0026] In operation, the VIS/NIR energy from the scene impinges upon thecorrector lens/aperture stop 110, and bounces off of the primarycomponent 115, which is preferably as large in diameter as the correctorlens 110. The energy then reflects onto and subsequently off of asecondary component 120 that is held in front of the primary component115, passes through the field lens 125 and forms an image on, forexample, an image intensifier (e.g., tube) 130. In this objectiveembodiment, primary and secondary components are both reflective, butthis need not always be the case. FIG. 1 illustrates the VIS/NIR imagingfunctionality of the multiple-band imaging system and FIG. 2 discussedbelow illustrates the MWIR and/or LWIR imaging functionality of themultiple-band imaging system.

[0027] Referring to FIG. 2, the objective lens 200 comprises acombination corrector lens/aperture stop 210, a primary component 215, asecondary component 220, and a MWIR or LWIR lens component 225.Components 210, 215 and 220 are identical to 110, 115 and 120 of FIG. 1,as these components form a single objective that is capable of imagingmultiple bands of radiation, e.g., VIS/NIR and MWIR and/or LWIR.

[0028] In operation, the MWIR or LWIR energy from the scene impingesupon the corrector lens/aperture stop 210, bounces off of the primarycomponent 215. In this embodiment, the primary component 215 diameter issimilar to the corrector lens aperture 210. The energy then reflectsonto and through a secondary component 220 that is held in front of theprimary component 215, passes through the MWIR or LWIR lens component225 and forms an image on, for example, an IR focal plane array (IRFPA)or the like 230. This objective is an example of a catadioptric system,wherein both reflective and refractive elements are used to achieve thedesired focal points. In this embodiment, the primary component 215 isreflective and the secondary component 220 is refractive.

[0029] In a first preferred embodiment illustrated in FIGS. 3(a) and3(b), by combining the elements of the objectives of FIG. 1 and FIG. 2,it is possible to simultaneously image the various energy bands from ascene using a single objective lens in a compact monocular 300.

[0030] Referring to FIG. 3(a), the components utilized during thedisplay of a night scene via a compact monocular 300 include acombination corrector lens (e.g., Cleartran®) 310, a primary component315, a secondary component 320, one or more MWIR or LWIR aberrationcorrection lenses 325, an IRFPA 330, one or more field lenses 335, animage intensifier 340, and a combining eyepiece (e.g., ANVIS-6 NV/HUD)345 which includes a image combiner 350, an eyepiece 355, and a display(e.g., active matrix electroluminescent (AMEL) display) 360. Alsoincluded in the compact sensor are thermal imager electronics andoperator controls (e.g., range focusing mechanism and polarity control)365. Imaging and sensor electronics are well know to those skilled inthe art and will not be discussed further herein.

[0031] In operation, all radiation 305, including visible through LWIRimpinges upon the combination corrector lens/aperture stop 310. Allradiation is reflected off of the primary component 315 and is directedtowards the secondary component 320. The secondary component 320 isdesigned so as to transmit either MWIR or LWIR or both, depending on thedesired application of the monocular, and to reflect VIS/NIR. Thetransmitted IR passes through an IR component 325 for correctingaberration and impinges upon an IRFPA, where it is converted into anelectronic digital image and then is displayed on, for example, a CRT,AMEL, or some other flat-panel display 360 and ultimately projected intothe eye 370 of a user via an imaging system 345.

[0032] The VIS/NIR wavelengths are reflected off of the secondarycomponent 320, pass through an aberration correction component 335 andare imaged onto an image intensifier 340 (e.g., II tube). After beingintensified, the VIS/NIR scene is directly imaged onto the user's eye370 via the imaging system 345. The imaging system 345 uses a beamcombiner 350 and an eyepiece 355 to superimpose the images onto theuser's eye 370. The eyepiece magnifies the output of the electronicallyintensified VIS/NIR scene and the digitized IR scene from the displayonto the viewer's eye 370.

[0033] Different colors must be used for the two images combined in theeyepiece 345. The output of the image intensifier 340 has thecharacteristic yellow green color of the P-43 phosphor screen. In orderto distinguish the overlaid IR image the output of the IR display 360must be a different color. The standard ANVIS-6 NV/HUD eyepiece uses anamber display for superimposing information onto the output of the imageintensifier. Thus monochrome amber is a good choice for displaying theIR image, but is merely exemplary.

[0034] Similarly, referring to FIG. 3(b), the components utilized duringthe display of a day scene via compact monocular 300 are identical tothose of FIG. 3(a) with the exception of the erector lens component 342which is used to image the visible scene in place of the imageintensifier 340 from the night scene.

[0035] The compact monocular 300 allows the user to choose betweenmultiple operation viewing modes, without the need to change objectiveor eyepiece lenses. The same scene, viewed in multiple bands (e.g.,VIS/NIR, MWIR, and LWIR), may be imaged through a single eyepiece at thesame magnification. Similarly, the user may view the same sceneseparately using at least two different imaging bands. In order toswitch between various modes of operation, a switch is provided. Thesemodes of operation include (1) only II image of the scene, (2) only IRimage of the scene, and (3) overlay of the II and IR image of the scene.Further, the monocular in FIG. 3(b) is equipped with optics for viewingscenes during the day without the need for the image intensifier.

[0036] The compact monocular 300 of FIG. 3 is preferably sealed to avoiddegradation due to exposure to the elements by the combinationaberration corrector lens 310. An example of a suitable sealing lens isone made of Cleartran®. While this is a well-recognized type ofcommercially available material, any lens fabricated from materials(e.g., zinc sulfide, zinc selenide) that transmit both visible, nearinfrared (NIR), MWIR, and LWIR may be used. One skilled in the artrecognizes that it is not always necessary to provide a sealingcomponent 310. In the case of larger aperture multiple-band imagers,sealing from outside elements may not be necessary, due to the size andcomposite of the components of the imagers and the strong likelihood theunit will remain in a protected location such as a perimeter tower. Theunit would be covered when not in use.

[0037] For the compact monocular 300 the objective lens has an f/numberless than f/1.5 for the image intensifier and closer to f/1.0 for theLWIR channel. In order to achieve this, there will likely be someperformance degradation at the edges of the field of view. However, thisis deemed acceptable within limits, in that the user of the compactmonocular 300 will center the object of interest, as is done withspotting scopes, binoculars, and even the human eye. Such degradationmay involve distortion (e.g., barrel or pincushion). Depending on theapplication the unit is designed to accommodate, the distortion of theobjective may be compensated in the eyepiece so that the net systemdistortion is zero. However, in either case, the distortion that ispresent must be nearly the same in both the image intensification andthermal channels for comfortable viewing of the images by the user.Also, the thermal image does not have to extend to the circumference ofthe image intensifier, but the magnification of the scene must be thesame.

[0038] In second and third preferred embodiments, shown in FIGS. 4(a)and 4(b), compact binoculars having multiple-band imaging capabilitiesare described. Referring to FIG. 4(a), compact binoculars 400 arecapable of imaging the VIS/NIR and MWIR or LWIR scenes onto one eye ofthe user. Also the MWIR or LWIR image can be displayed to the other eye.The same displays are used with the full daylight imaging optics suchthat the compact binocular 400 is useful during both day and night. Inthis example, we will refer to the two bands of energy as VIS/NIR andLWIR. The compact binocular 400 comprises an objective lens 405 whichincludes a combination corrector/sealing lens (e.g., Cleartran®) 410, aprimary component 415, and a secondary component 420. Compact binocular400 further comprises a beam splitter 425, an IRFPA 430, an aberrationcorrection lens 435, image intensifier 440, a first display screen 445,a switchable holographic reflector/filter 450, a first eyepiece 455, asecond eyepiece 460, a second display screen 465, a charge-coupleddevice (CCD) 470, a zoom lens system 475, and thermal imager electronics480.

[0039] In this second preferred embodiment, the LWIR and the VIS/NIR areimaged onto one eye of the viewer 490, with the LWIR image onlyavailable to the other eye 485. Both energy bands impinge upon objectivelens 405 and the VIS/NIR is reflected (thus maintaining the resolution)and the LWIR energy is passed through a beam splitter 425 appropriatelycoated for reflection and transmission in the selected energy bands. Thetransmitted LWIR energy is detected by IRFPA 430 and is imaged viathermal imaging electronics 480 through a display (e.g., AMEL) 445 andsubsequently, eyepiece 460 onto the viewer's eye 485. Though the LWIRimage quality passing through the beam splitter 425 is somewhatdegraded, the large pixels are less impacted by this fact.

[0040] The reflected VIS/NIR passes through image intensifier 440,mounted perpendicular to the line of sight. The image intensifiedVIS/NIR radiation is reflected by holographic filter 450, passes througheyepiece 455 and projects the image into the viewer's eye 490. The LWIRimage is displayed 465 and passes through the holographic filter 450,combining with the VIS/NIR image in the eyepiece 455 and projects itinto the viewer's eye 490.

[0041] Given the binocular configuration that results from the secondpreferred embodiment, a day visible camera could also be included to bedisplayed on both display screens 465 and 445. In this case, day zoomoptics 475 act upon impinging visible light which is collected by a CCDand subsequently imaged onto the displays (e.g., AMEL) 465 and 445. Forthis visible scene (e.g., daylight scenario), the holographic filter 450is switched so as to transmit all visible light, which passes througheyepiece 455 and impinges upon the viewer's eye 490.

[0042] As is apparent from the previous discussion, in FIG. 4(a), theswitchable holographic filter 450 may be switched from nighttime modewhere it reflects the P-43 yellow-green and transmits the selected falsecolor of the LWIR sensor to the daytime mode where it has no opticaleffect and allows the visible to pass through to achieve a full colordisplay. Using the switchable holographic filter 450 negates the needfor mechanically moving the element in and out of the beam path.Alternatively, the switchable holographic filter 450 may be a simplebeam combiner which at night reflects the P-43 yellow-green andtransmits the selected false color of the LWIR sensor. During the daythe beam combiner is moved out of the way to achieve a full colordisplay.

[0043] In this second preferred embodiment, the first objective lensformed by at least the primary and secondary components 415 and 420, theII tube 440, and right eyepiece 460 must be fixed, the left eyepiece 455can be moved to adjust for different users.

[0044] Referring to FIG. 4(b), an alternative configuration for acompact binocular 500 is shown. Compact binocular 500 images at leasttwo separate energy bands. In this particular example, the two energybands are LWIR and VIS/NIR. In particular, compact binoculars 500,project the image of the LWIR into both eyes 585, 590 of the viewer andimages the VIS/NIR onto one eye 590 of the user. In order to achievethis dual imaging, the binoculars 500 utilize two separate objectivelenses 505 and 605, one in each eye channel. Each objective lens 505,605 is optimized for optical performance in its respective energy band.In FIG. 4(b), objective lens 505 is optimized for the VIS/NIR energyband, while objective lens 605 is optimized for the LWIR energy band.

[0045] After passing through the objective lens 505, VIS/NIR energy isreflected off of a prism (e.g., right angle prism) 525 and passesthrough an image intensifier 540. The intensified VIS/NIR energyimpinges upon image combiner 550, where the intensified VIS/NIR iscombined with the LWIR (described later), is reflected off of thecombiner 550, passes through eyepiece 555 and into the viewer's eye 590.

[0046] Similarly, after passing through objective lens 605, LWIR energyimpinges upon IR detector (e.g., IRFPA) 530 and is imaged onto display(e.g., AMEL) 565. The LWIR scene imaged from the display 565 istransmitted through image combiner 550, is combined with the VIS/NIRimage, passes through eyepiece 555 and into the viewer's eye 590. At thesame time that the IR detected radiation is being imaged on display 565,it is simultaneously being imaged onto display 545, where the LWIR imagepasses through eyepiece 560 and into viewer's eye 585. Thermal imagingelectronics 580 control the imaging displays 545 and 565.

[0047] Compact binoculars 500 offer a number of viewing advantages tothe user including, superimposition of the dual-band images to assureregistration, maximum control of the image intensifier channel, andmultiple-band imaging in one or both eyes.

[0048] In a fourth preferred embodiment of the present invention, thevisible band is again imaged with at least one infrared band i.e., MWIRand LWIR, using a single objective lens configuration. In the fourthpreferred embodiment, the visible band is not intensified electronicallyusing an II tube and then imaged. Instead, the scene is imaged directlyusing available visible light in full color and displayed to the userlike a standard handheld monocular telescope. The selected IR band isdisplayed monochromatically as an overlay in the eyepiece (thebrightness of the wavelength used in the IR overlay may have to bereduced in the visible channel with a notch filter to enhance theoverlay). The imaging scenario described above is achieved using thelens monocular configuration of FIG. 3(b), which depicts a visible/IRband imaging system 300 for simultaneously imaging full color visiblewith the selected IR band. The system 300 is comprised of a combinationcorrector lens (e.g., Cleartran®) 310, a primary component 315, asecondary component 320, an IR aberration correction lens 325, an IRFPA330, field/image inverting lenses 335, an image erector component 342,and an imaging system 345 which includes an image combiner 350, aneyepiece 355, and a display (e.g., active matrix electroluminescent(AMEL) display) 360. Also included in the compact sensor are thermalimager electronics and controls 365.

[0049] In operation, all radiation bands 305 from a scene impinge uponthe combination corrector lens/aperture stop 310 and reflect off of theprimary component 315 towards the secondary component 320. The visibleradiation component of the incoming radiation 305 reflects off of thesecondary component 320 and passes through the remainder of the systemand is projected into the eye 370 of the user. The IR radiationcomponent of the incoming radiation 305 passes through the secondarycomponent 320 and an IR aberration correction lens 325 and onto an IRFPA330. The IRFPA digitally converts the IR radiation information throughthermal control electronics 365 and images the IR scene through an AMEL360. The converted IR monochromatic image from the AMEL 360 and thedirect view full color visible image are combined at image combiner 350and imaged by eyepiece 355 onto the user's eye 370.

[0050] For both the first and fourth preferred embodiments, using acompact 170 mm focal length catadioptric and the standard eyepiece, theresult will be approximately a 6 degree field of regard for the imageintensifier and likely somewhat less for LWIR. The second and thirdembodiments can be used with any combination of lenses in a handheldbinocular configuration. Further, a likely configuration for the thirdembodiment is a helmet or head mounted goggles. In this case the lensand eyepiece focal lengths are selected so that the output is unitymagnification. In this way the operator can walk, run and operateequipment normally. The field of regard for such a configuration is inthe range of 40 to 60 degrees.

[0051] A standard 18 mm image intensifier tube is used in the preferredembodiments, but one skilled in the art recognizes that this parametermay vary depending on the desired size of the system and other systemdesign requirements. Further, multiple IRFPA formats may be usedincluding, but not limited to 160×120 and 320×240 with 46 to 51 micronpitch, and 320×240, and 640×480 with 25 to 28 micron pitch. The overallmagnification of the first two embodiments described above isapproximately a power of 7. The preferred embodiments of the presentinvention further contain various controls including on/off, rangefocus, LWIR polarity, and II/LWIR/overlay selection. To further exploitthe resolution of the image tube, eyepiece zoom magnification may beincluded. Additionally, due to the compact nature of the systems it ispossible to include auxiliary sensors such as a magnetic compass, GPSreceiver and a range finder within the housing of the system. The datafrom these sensors may be provided to the user via, for example, theAMEL display regardless of whether or not the LWIR or MWIR image isselected. The IR digital video is also suitable for recording or relayusing, for example, an RS-232 port to gain access to the thermal digitalvideo.

[0052] While the embodiments discussed above are directed specificallyto the design of a handheld multiple-band imager operating in the130-200 mm focal length range, similar design approaches may be used tofor special reconnaissance hide-sight imagers operating in the 250-350mm focal length range and platform mounted imagers operating in the500-700 mm focal length range. The different focal length requirementswill result in the need for different objective lens components, but thesame design approach of reflecting the visible and transmitting the LWIRthrough the secondary component is still applicable. In operation, alonger focal length results in the following trends (a) biggerobscuration formed by the secondary component, (b) easier packaging, (c)more expensive components due to their size. Generally, the LWIR driveelectronics, image intensifier, housing, controls and eyepiece willremain the same for each focal length range.

[0053] The preferred embodiments discussed above are based on anexisting uncooled IRFPA technology, specifically for the handheldversion the IRFPA is 160×120 or 320×240 with a 25 to 51 micron pitch(e.g., supplied by Boeing, Lockheed Martin and Raytheon), withcorresponding drive electronics. The IRFPA is located such that theavailable array can be removed and a new array positioned at the samelocation for demonstration and comparison. As IRFPA technology improves,the imagers can also be designed so as to be used as a test-bed for anynew FPA configurations, as the arrays and drive electronics becomeavailable. The multiple-band imaging devices described herein may bepowered by any conventional source, self-contained or otherwise,including, for example, a battery pack.

[0054] Returning to the objective lens designs contemplated by theinvention (See FIGS. 1 and 2), for a monochromatic situation (onewavelength for the visible and the LWIR bands), the first-orderparameters for a two-mirror objective have been explored to determinethe acceptable power distribution in the primary and secondary mirrors.By way of example, a preferred design is f/1.4 with a 121 mm entranceaperture (170 mm focal length), mirror separation of approximately 58.5mm, linear obscuration of approximately 0.30 (36 mm diameter), and theII tube protruding into the mirror system. An aperture stop with acorrector element is located in front of the two-mirror system to aid inthe control of aperture aberrations (it also seals the unit from theoutside environment). One or more field lenses near the II tube areneeded to aid in the control of field aberrations. Some monochromaticperformance results for the objective lens of FIG. 1 are illustrated inFIGS. 5(a) and 5(b). FIG. 5(a) illustrates the Modulation TransferFunction (MTF) plot of the feasibility design for the II channel of theobjective lens. As indicated in the FIG. 5(a), the performance of thefeasibility design across the field of view (II tube format) is nearlydiffraction limited for all spatial frequencies from the effectiveNyquist frequency of the II tube on down. In other words, theoptics-detector combination is detector limited in this monochromaticdesign analysis. This high level of monochromatic performance in thedesign establishes the feasibility of creating high quality imagerywhile leaving room for achromatizing it over the required spectral band,lens design simplification, fabrication tolerances, and other suchtradeoffs.

[0055] The astigmatism and distortion of the II channel lens design areindicated in FIG. 5(b). The smooth monotonic nature of the astigmatismcurves indicate that the lens design minimizes field aberration and thatthere is no higher-order aberration balancing occurring in order toachieve the high level of performance indicated in FIG. 5(a). Thedistortion curve is also monotonic and has a low magnitude out to theedge of the field.

[0056] Referring to FIGS. 6(a) and 6(b), the lens configuration of theLWIR channel of FIG. 2 is also explored with a monochromatic lensdesign. FIG. 6(a) illustrates the MTF plot of the design for the LWIRchannel. The performance is very good on axis and over most of the fieldof view, relative to the diffraction limit, with a fall off at thecorner of the detector. Due to the fact that the user will only use thefield for acquisition over the scene being viewed and then bring theobject of interest to the center of the field of view, this drop off inperformance at the corner is not considered a significant detriment inthis feasibility design.

[0057] The astigmatism and distortion of the LWIR channel lens designare indicated in FIG. 6(b). The smooth nature of the astigmatism curvesindicates that the design does not have higher-order field aberrations.The distortion curve is monotonic and has a low magnitude out to theedge of the field. This result indicates that controlling of thedistortion of the LWIR channel with respect to the II channel isachievable to a close agreement level between the two channels. In orderto optimize the objective lens design for a multiple-band imager,multiple parameters may be required and/or considered.

[0058] Basic optical parameters of the multi-band objective lens includethe effective focal length (EFL) which is preferably between 130 and 200mm; relative aperture (f/n) which is preferably 1.5 to 1.0 or faster forthe IR channel(s); pupil diameter which is, for example, 121 mm for EFLof 170 mm, pupil linear obscuration minimized to between approximately0.25 and 0.3, wavelength band imaging between 0.6 to 14 μm, andselection of detectors for both the VIS/NIR, the MWIR, and the LWIR.

[0059] Optical performance parameters which need to be maximized in thelens design of the multiband objective lens besides the imagery includereflection/transmission per surface, transmission per lens system,relative illumination uniformity. Optical performance parameters thatare to be minimized include ghosts, distortion, and any vignetting.

[0060] To optimize the optical performance (e.g., minimize chromaticaberration and other distortions) of the objective lens, the sphericityor asphericity of the corrector lens, primary component, and/orsecondary components is a parameter that may be manipulated. Dependingon the intended end-use for the multiple-band imaging device, at leastone of these components may be aspheric in order to meet specificoptical criteria. Additionally, the use of different materials for theindividual lenses comprising the corrector lens and the primary andsecondary components, the use of a diffractive optical element (DOE)within the components, as well as the curvatures of the individual lens(e.g., positive, negative) may also be considered in order to optimizeoptical performance of the objective lens. Further, field lenses tocorrect for aberrations may also be used.

[0061] By way of example, other possible VIS/NIR optical configurationsfor this application from the monochromatic performance are shown inFIGS. 7(a) through 7(d). These embodiments vary based on the number andlocation of aspheric surfaces. In FIG. 7(a) all three major opticalcomponents the corrector lens 10, the primary component 15, and thesecondary component 20 of the objective lens have aspheric surfaces. InFIG. 7(b) the corrector lens 10 and the secondary component 20 haveaspheric surfaces. In FIG. 7(c) the corrector lens 10 and the primarycomponent 15 have aspheric surfaces and in FIG. 7(d) only the correctorlens 10 has an aspheric surface.

[0062] Referring to FIGS. 8(a) through 8(f), color-corrected lens designforms for the VIS/NIR band from 600-1,000 nm are presented. In FIG.8(a), the primary and secondary components 15, 20 are spherical whilethe corrector lens 10 is aspheric and includes a DOE. In FIGS. 8(b) and8(c), the corrector lens 10 is comprised of two lenses 11 and 12, havingin order of incoming scene interaction, negative and positive overallpower, respectively. All of the components, including the lenses 11, 12and the primary and secondary components 15, 20 are spherical. Furtherthe objective lens in FIG. 8(b) includes an aperture stop 21 locatedapproximately midway between the corrector lens group and the primarycomponent 15. In FIGS. 8(d) and 8(e), the corrector lens 10 is comprisedof two lenses 11 and 12, having in order of incoming scene interaction,positive and negative overall power, respectively. All of thecomponents, including the lenses 11, 12 and the primary and secondarycomponents 15, 20 are spherical. By way of example, FIG. 8(g)illustrates the color-corrected modulation transfer function (“MTF”)performance available to that of lens configuration depicted in FIG.8(e) for the II channel. As depicted, performance is maximized in thecenter of the format where the device will be primarily be used.Additionally, the monochromatic lens design forms presented earlier canbe designed to be color-corrected over the intended spectral bandyielding a wide array of lens design choices to fulfill the specificdesign goals of the multi-band device. An alternate lens design formthat is scalable to long focal length applications does not include anaperture corrector(s) in front of the objective. Instead, the aberrationcorrection of these alternate lens designs is achieved with a morecomplex field lens group near the II tube input faceplate, see FIG.8(f). For shorter focal length versions of this form, a thin,flat-surfaced window of appropriate material (e.g., zinc sulfide, zincselenide) can be used to seal this more compact unit from the outsideenvironment. By way of example, FIG. 8(h) illustrates thecolor-corrected MTF performance available to that of the lensconfiguration depicted in FIG. 8(f) for the II channel.

[0063] Referring to FIGS. 9(a) and 9(b), the LWIR band opticalperformance is also acceptable using the objective lens configurationsof FIGS. 8(b) and 8(e), respectively. In FIG. 9(a), the three majoroptical components 10, 15, and 20 of the objective lens are allspherical. In the LWIR band, the radiation passes through the secondarycomponent 20 unlike the VIS/NIR band, which is reflected by thesecondary component 20. Similarly, in FIG. 9(b), the three major opticalcomponents 10, 15, and 20 of the objective lens are all spherical andthe radiation passes through the secondary component 20. In FIGS. 10(a)and 10(d), the all-spherical objective lens configurations of FIGS.8(b), 8(e), 9(a), and 9(b) are shown in combination, including theVIS/NIR and LWIR detectors 30 and 40. Further, FIGS. 10(b), 10(c),10(e), and 10(f) depict the MTF performance for both the VIS/NIR and theLWIR components of comprising FIGS. 10(a) and 10(d), respectively. Alsoincluded in all of the FIGS. 7(a) through 10(f), are supporting opticalcomponents (e.g. aberration correction lenses) (not numbered) for boththe VIS/NIR and the LWIR bands. These supporting optical components neednot be specifically described herein as they are easily understood bythose skilled in the art.

[0064] The IR lenses described above may be formed from any suitablematerial (e.g., zinc sulfide, zinc sulfide MS, zinc selenide, germanium)as is commonly understood by those skilled in the art. One skilled inthe art readily understands that with the exception of those parametersdetailed above, the optical parameters i.e., radius of curvature, indexof refraction, diameter, material, etc., of all of the lenses includingcorrection, field, primary, and secondary, used in the embodiments ofthe present invention are selectable based on the specific designrequirements of each imaging system. As such, these particulars will notbe discussed further herein.

[0065] The environmental parameters that are met by the preferredembodiments include those applicable to a handheld device used bymilitary personnel, including temperature operational and storage range,shock, vibration, humidity and altitude. Commercial variations of theseenvironmental parameters may be less demanding. The housing requirementsmet by the objective lens of the preferred embodiments include a sealed,protective front element and the capability for back-filling with a dryinert gas.

[0066] Packaging considerations taken into account in designing thepreferred objective lenses of the present invention include filters andfilter location with the objective system, window location, material,thickness distance from primary component, minimization of the number ofelements comprising the objective, number of different optical materialsused to compose the objective, types of surfaces (e.g., aspheric),length, diameter, and weight.

[0067] In order to allow for simultaneous range focusing, a preferredapproach contemplated by this invention is to move the primary componentin relationship to the secondary component, a technique that is used inother catadioptric lens applications. In this manner both wavebands willbe adjusted for range. Alternatively, a second approach is to move theentire lens in relationship to the II tube and use a follower mechanism(mechanical or electrically driven) for the LWIR section. A thirdapproach is to move the field lens(es) in front of the II tube and afollower mechanism in the LWIR optical path.

[0068] In order to keep multiple channels in simultaneous focus over thefull temperature range, selection of materials for the housing andmounting brackets is made to achieve uniform focus over the range. Analternate to housing material selection for uniform focus over fulltemperature range is to use an electronically controlled thermal focusadjustment to compensate for discrepancies between channels whenstandard housing materials are used.

[0069] The applications for this type of multiple-band imaging systemare numerous. To take a specific example, a firefighter, equipped withthe visible/IR imaging system above, is able to go from daylight into asmoke-filled building and vice versa and maintain some amount of visualcontact without ever removing the imaging system or having to switchbetween multiple imaging systems or components. Further, the firefighterneed not choose between visible and IR, but may simultaneously maximizehis/her viewing capability by utilizing-both views at all times. Otherapplicable users include all emergency personnel (e.g., police, search &rescue), the military, sportsman (e.g., hunters), ship operators,aircraft operators, and the like. This type of compact imaging system isuseful in any situation where there could be a fluctuation or shift fromvisible to IR viewing conditions and vice versa.

We claim:
 1. An objective lens system for imaging at least two bands ofincoming radiation comprising: a first optical element for reflectingthe at least two bands of incoming radiation; and a second opticalelement for reflecting a first of the at least two bands of radiationand transmitting a second of the at least two bands of radiation,wherein the at least two bands of incoming radiation are selected fromthe following group consisting of the visible band, the VIS/NIR band,the mid-wavelength infrared band and the long-wavelength infrared band.2. The objective lens according to claim 1, further comprising acorrective element for correcting aberrations in the at least two bandsof incoming radiation
 3. The objective lens according to claim 2,wherein the corrective element comprises: a first lens having positiverefractive power; and a second lens having negative refractive power. 4.The objective lens according to claim 2, wherein the corrective elementcomprises: a first lens having a negative refractive power; and a secondlens having a positive refractive power.
 5. The objective lens accordingto claim 3, wherein the corrective element, including the first andsecond lenses, the first optical component, and the second opticalcomponent are comprised of spherical surfaces.
 6. The objective lensaccording to claim 4, wherein the corrective element, including thefirst and second lenses, the first optical component, and the secondoptical component are comprised of spherical surfaces.
 7. The objectivelens according to claim 3, wherein the corrective element, including thefirst and second lenses, the first optical component, and the secondoptical component are comprised of aspherical surfaces.
 8. The objectivelens according to claim 4, wherein the corrective element, including thefirst and second lenses, the first optical component, and the secondoptical component are comprised of aspherical surfaces.
 9. The objectivelens according to claim 3, wherein at least one of the first and secondlenses of the corrective element, the first optical component, and thesecond optical component includes at least one aspherical surface. 10.The objective lens according to claim 4, wherein at least one of thefirst and second lenses of the corrective element, the first opticalcomponent, and the second optical component includes at least oneaspherical surface.
 11. A multiple-band radiation imaging device forimaging at least one scene onto at least one eye of a viewer comprising:an objective lens for imaging at least two bands of incoming radiationindicative of a scene; a first sensor for receiving a first of the atleast two bands of incoming radiation; and a second sensor for receivinga second of the at least two bands of incoming radiation, wherein thefirst sensor images a first scene formed from the first of the at leasttwo bands of incoming radiation on at least one display device and asecond sensor increases the intensity of a second scene formed from thesecond of the at least two bands of incoming radiation prior to imagingat least one of the first and second scene onto the at least one eye ofthe viewer.
 12. The multiple-band radiation imaging device of claim 11,wherein the first of the at least two bands of incoming radiation islong-wavelength infrared radiation.
 13. The multiple-band radiationimaging device of claim 11, wherein the first of the at least two bandsof incoming radiation is mid-wavelength infrared radiation.
 14. Themultiple-band radiation imaging device of claim 11, wherein the secondof the at least two bands of incoming radiation is VIS/NIR radiation.15. The multiple-band radiation imaging device of claim 11, wherein thefirst sensor is an uncooled infrared focal plane array.
 16. Themultiple-band radiation imaging device of claim 11, wherein the secondsensor is an image intensification device.
 17. The multiple-bandradiation imaging device of claim 11, wherein the objective lenscomprises: a first optical element for reflecting the at least two bandsof incoming radiation; and a second optical element for reflecting afirst of the at least two bands of radiation and transmitting a secondof the at least two bands of radiation, wherein the at least two bandsof incoming radiation are selected from the following group consistingof the visible band, the near-wavelength infrared band, themid-wavelength infrared band and the long-wavelength infrared band. 18.The multiple-band radiation imaging device of claim 11, furthercomprising a scene switch, wherein the viewer can switch between thefollowing viewing modes: (a) the first scene only; (b) the second sceneonly; and (c) a superposition of the first scene and the second scene.19. The multiple-band radiation imaging device of claim 17, wherein theobjective lens further comprises a corrective element for correctingaberrations in the at least two bands of incoming radiation, wherein thecorrective element is located first in the optical path. 20.Multiple-band radiation imaging binoculars for imaging at least twobands of radiation onto at least one eye of a viewer comprising: a firstobjective lens for imaging a first band of incoming radiation, whereinthe first band of incoming radiation passes through the at least oneobjective lens in a first direction; a first optical element forreceiving the first band of incoming radiation and redirecting the firstband of radiation in a second direction, wherein the second direction isat approximately a 90 degree angle to the first direction; a firstsensor for receiving the first band of incoming radiation in the seconddirection, wherein the first sensor intensifies the first band ofincoming radiation; a second optical element for receiving theintensified first band of incoming radiation and redirecting theintensified first band of radiation in a third direction, wherein thethird direction is at approximately a 90 degree angle to the seconddirection and is the same direction as the first direction; a firsteyepiece for imaging the intensified first band of radiation and thesecond band onto a first eye of the viewer; a second objective lens forimaging a second band of incoming radiation, wherein the second band ofincoming radiation passes through the second objective lens in the firstdirection; a second sensor for receiving the second band of incomingradiation, wherein the second sensor images the second band of incomingradiation on a first and a second display device, wherein the secondband of incoming radiation is imaged from the first display device ontothe first eye of the viewer via the first eyepiece; a second eyepiecelens for imaging the second band of incoming radiation from the seconddisplay device onto a second eye of the viewer, such that the first bandof incoming radiation is imaged onto one eye of the viewer and thesecond band of incoming radiation is imaged onto both eyes of theviewer.
 21. The multiple-band radiation imaging binoculars of claim 20,wherein the first band of incoming radiation is VIS/NIR radiation. 22.The multiple-band radiation imaging binoculars of claim 20, wherein thesecond band of incoming radiation is mid-wavelength infrared radiation.23. The multiple-band radiation imaging binoculars of claim 20, whereinthe second band of incoming radiation is long-wavelength infraredradiation.
 24. The multiple-band radiation imaging binoculars of claim20, wherein the second sensor is an uncooled infrared focal plane array.25. The multiple-band radiation imaging binoculars of claim 20, whereinthe second sensor is an image intensification device.
 26. Amultiple-band radiation imaging device for imaging at least one sceneonto at least one eye of a viewer comprising: an objective lens forimaging at least two bands of incoming radiation indicative of a scene;a first sensor for receiving a first of the at least two bands ofincoming radiation, wherein the first sensor images a first scene formedfrom the first of the at least two bands of incoming radiation on atleast one display device which is then viewed by the at least one eye ofthe viewer; and a second sensor for receiving a second of the at leasttwo bands of incoming radiation, wherein the second sensor is the atleast one eye of the viewer.